<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <style>

        @media screen and (max-width: 900px){
            @import "600.css";
        }
        @media screen and (max-device-width: 1920px){
            .box {
                width: 700px;
                height: 500px;
                font-size: 18px;
                padding: 10px;
                background-color: red;
            }
            html{
                font-size: 62.5%;
            }
            .boxs{
                width: 700px;
                height: 500px;
                font-size: 1rem;
                padding: 10px;
                background-color: red;
            }
        }
    </style>
</head>

<body>
<div class="box">
    像素的单位
</div>
<div class="boxs">
    rem单位
</div>
</body>
<script>

</script>

</html>